skin-effect
(rus. скин-эффект)
— phenomenon of electromagnetic waves decay as they penetrate into a conducting medium.

Description

An alternating electric field and associated magnetic field do not penetrate into the conductor but are concentrated mainly in a relatively thin surface layer (the so-called skin layer). The nature of the skin effect is explained as follows: free electrons in the conductor, under the impact of an external alternating field, create currents which compensate the external field within the conductor's volume (skin effect occurs in metals, plasma, ionosphere, degenerative semiconductors and other media with sufficiently high conductivity).

The skin layer depth depends to a great extent on the sample's conductivity, electromagnetic field frequency and surface condition. Being sufficiently high at low frequencies, the skin layer's depth decreases with the frequency rise ; the skin layer's depth of metals, at frequencies of optical range, becomes comparable to the wave length (it is precisely the small penetration of the electromagnetic field and almost its total reflection that explains metallic luster of good conductors). For example, the skin layer depth in copper wire at an electromagnetic field frequency of 50 Hz (standard frequency for the "urban" current) is about 1 cm, it is about 0.1 cm at a frequency of 5 kHz, and it is about 10 microns at a frequency of 0.5 MHz.

Sometimes the situations take place when electrons' mean free path exceeds the skin layer's thickness; it is said that an anomalous skin effect takes place (it occurs when pure metals are exposed to microwave electromagnetic field at low temperatures). Under such an effect scattering of electrons on the sample's surface does not affect much the skin layer's thickness (here, a significant role is played by electrons with small glancing angles for which reflection is close to specular reflection).

At sufficiently high intensity of alternating electromagnetic field, when the medium's parameters, such as conductivity, are beginning to depend on the field, the skin effect becomes nonlinear, i.e. the skin layer's depth also begins to depend on the electromagnetic field intensity (the nonlinear skin effect is most easily realised in plasma). The threshold values of electromagnetic field amplitudes at which skin effect transits into the nonlinear phase depend on the parameters of the medium and electromagnetic field frequencies.